Covers the study of (mostly homogeneous) macroscopic systems from a heat/energy/entropy point of view. Maybe combine with [tag:statistical-mechanics].
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69 views
thermodynamics of a dual-face surface in space
This question is a continuation from this one.
A material disk have two sides, one that is reflective and another absorptive of electromagnetic radiation in the range where the background cosmic ...
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4answers
456 views
Why the breath sometimes warm and sometimes cold? (2 different explanations!)
If you blow air against your hand with your mouth open, you feel warm breath. If you do with with your lips closed except for a small opening, you feel cold breath.
One explanation from here says ...
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1answer
729 views
What is the relationship between Energy, Entropy, and Information?
What is the relationship between Energy, Entropy, and Information?
I read this - What Is Energy? Where did it come from? - and the top answer says that 'energy' is an abstract number that is a ...
4
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2answers
282 views
What causes a Phase-Transition
A phase transition occurs when for example, heat is applied continuously to a liquid and after a certain time it converts into a gas.
How does this process work in detail? Is their a chain reaction ...
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3answers
279 views
Thermodynamics of cooking rice
The question is whether (1) cooking rice in a pot of boiling water for an hour and then leaving the pot (of water and rice) to cool to room temp heats my house the same amount as compared to (2) just ...
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1answer
3k views
Why is my tupperware sealed after heating it up in the microwave?
I had a vegetable soup in a crystal tupperware and I put it in the microwave to heat it up.
The funny thing is that when I took it out, it was impossible to open it up. The lid is bent towards the ...
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1answer
171 views
Voltage drop over a cell membrane
Again, a problem from exam preparation:
[A] cell's membrane allows sodium ions to pass through it, but not chlorine ions. The cell is placed in a salty solution with a ten times higher ...
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1answer
501 views
Thermodynamics of evaporation
If water is introduced in a container maintained at 20 °C in vacuum conditions, a gaseous phase will appear and the pressure will stabilize at the vapour pressure for the given temperature inside the ...
4
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1answer
140 views
Boltzman distribution for chemical potentials
I read that if we have a system with two co-existing phases with chemical potentials respectively $\mu_1$ and $\mu_2$ then, at the equilibrium, the concentrations $X_1$ and $X_2$ are related by the ...
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4answers
612 views
Decompressing gas whilst driving a turbine - where does the energy come from?
Here's the situation: a friend is working on a civil engineering project to make some equipment to decompress liquefied natural gas before pumping to households. I don't know what the proper term for ...
4
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1answer
223 views
How to estimate condensation from air?
How to estimate the amount of water condensing from air on a surface, given the air's temperature and relative humidity and how they change over time, the surface temperature, material's thermal ...
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0answers
79 views
Mechanical Equivalent of Heat
Recently I have been looking up James Joule's experiment regarding the mechanical equivalent of heat. After viewing some drawings of the apparatus, I assumed that the lines holding the weights would ...
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0answers
122 views
Thermodynamic relations from Gibbs-Duhem
Given the Gibbs-Duhem relation
$V dp = S dT - N d \mu$, I am having trouble deriving the following identity:
$\ (\frac{\partial N}{\partial \mu})_{V,T} = N (\frac{\partial \rho}{\partial p})_T$
...
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1answer
327 views
What meaning does the slope of the efficiency path on a Mollier diagram have in terms of temperature?
Let's say I have a steam turbine that I have modified to increase its isentropic efficiency. As a specific case, consider the modification outlined in the Mollier diagram below. The arrows represent ...
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0answers
215 views
What's the efficency of a steam jet pump?
Jet pumps or venturi pumps are often stated as having a "terribly low" efficiency, steam jet pumps specifically are usually describes as "only justifiable when there's an abundant steam supply anyway" ...
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5answers
8k views
Which direction does air flow?
I remember learning this in high school, but have forgotten it, and can't seem to find it anywhere online.
Air travels from areas of high pressure to low pressure...correct? So if I have a cold room ...
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0answers
777 views
Inflating a balloon (expansion resistance)
I am doing a quick calculation on how to calculate the pressure needed to inflate a perfectly spherical balloon to a certain volume, however I have difficulties with the fact that the balloon (rubber) ...
4
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2answers
202 views
Why are the lighter halogens gases?
In the periodic table of the elements, only a few of the elements are gases at standard temperature and pressure. Those elements include the noble gases some of the halogens, and a few of the elements ...
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5answers
507 views
Crash course in classical thermodynamics
I've been working on some projects lately where it would be very handy to know more about thermodynamics than I do, but sadly I never had a chance to take a proper thermodynamics course in college. ...
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3answers
800 views
Could life survive a pole shift caused by an asteroid collision?
Could life on earth survive a large pole shift caused by an asteroid collision?
I became aware that there are people who believe that the earth's pole suddenly shifts. That is, its rotational ...
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4answers
537 views
2nd Law of Thermodynamics
I understand that the 2nd law of thermodynamics roughly states that, if you have a body (or a gas in a chamber) that is hot at one end and cold on the other, the heat will always flow from the hot to ...
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5answers
316 views
What is the meaning of following expresion $C=\frac{\delta Q}{dT}$ mathematicly
Our professor raised the following question during our lecture in Statistical Physics (even so it's related to Thermodynamics):
Many text books (even wikipedia) writes wrong expressions (from ...
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3answers
4k views
Why does microwaved food get cold faster
My evidence is entirely anectodal and non-scientific, but I've noticed food gets cold faster when it's been heated in a microwave instead of a stove. Is this true? And if it is, why does it happen?
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2answers
315 views
Heuristics for specific heat capacities of solids
A didactic question publish in The Physics Teacher (http://tpt.aapt.org/resource/1/phteah/v41/i1/p8_s1) asks which will melt more ice: 100g of metal at 100C or 100g of wood at 100C. (The particular ...
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3answers
332 views
Proof of existence of lowest temperature $0 K$
Im mathematics there is a concept of infinity meaning that whenever you pick a number and say that it is the smallest/Largest there is a way to further reduce/increase that number by ...
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5answers
1k views
Second law of Thermodynamics: Why is it only “almost” always true that entropy is non-decreasing?
The venerable Wikipedia states the second law of Thermodynamics as such:
...
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3answers
1k views
Does opening the oven door heat the house more than if the door is closed?
In the winter I am in the habit of opening my oven door once I am done baking so that I can add the heat to the house. However I recently thought about it and it would seem that even if the door is ...
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4answers
523 views
Why water inside a vessel placed in a body of boiling water does not boil?
Imagine a plastic sac filled with water placed in a pan of boiling water. I wonder why the water inside the sac does not boil. The temperature of the water inside the sac should be the same as that of ...
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4answers
1k views
Heat in the car during sunny day
Why is it hotter inside an isolated car (air conditioning is off) than outside during a sunny day in summer?
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4answers
1k views
Does a full freezer freeze items faster than an empty one?
Let's say we want to freeze a banana. Would is freeze faster if we insert it into an empty freezer, or into one that already contains many frozen bananas?
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1answer
375 views
heater in a perfectly insulated box
Imagine a perfectly insulated box, placed inside the box is an electric heater. The heater is switched on and the box is left to reach equilibrium with its surroundings. What is the final temperature ...
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3answers
306 views
How to think physically about basic “fields”
"Field" is a name for associating a value with each point in space. This value can be a scalar, vector or tensor etc. I read the wikipedia article and got that much, but then it goes it into more ...
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1answer
91 views
Why does $U = T S - P V + \sum_i \mu_i N_i$?
From the first law of thermodynamics:
$$ dU = TdS - PdV + \sum_i\mu_i dN_i.$$
Quoting Wikipedia:
As conjugate variables to the composition $N_{i}$ the chemical potentials are intensive ...
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3answers
193 views
Irreversible process
I have this problem. I have an ideal gas that goes through an irreversible adiabatic decompression. I have the initial state (P,T,V), and the final pressure, and I have to calculate the entropy ...
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2answers
163 views
Why do hot objects prefer to emit photons over electrons ? Is there electron-positron annihilation?
Why do hot objects prefer to emit photons over electrons ? Is there electron-positron annihilation ? If so , why ? Im confused by this.
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3answers
231 views
Fast cool down of “things” in the kitchen
Is there any fundamental physical reason (thermodynamics/entropy?) behind the fact that there doesn't exist home appliance for fast cool down of food/drinks?
I know that there are some methods ...
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3answers
230 views
Do I have the meaning of the property temperature correct?
OKay my book just starts out talking about the vague definition we have for temperature and we ended up with the Zeroth law of Thermodynamics which states:
Two systems are in thermal equilibrium ...
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4answers
618 views
What are the arguments towards the Life-and-Entropy relation?
I've heard it from a few people, and I've seen it popup here in the site a couple of times. There seems to be speculation (and studies?) towards this idea, and this is what I've picked up so far:
...
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2answers
415 views
Law for tap water temperature
I was wondering if anyone put together a law to describe the rising temperature of the water coming out of a tap.
The setup is fairly simple: there's a water tank at temperature T, a metal tube of ...
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3answers
161 views
What are some creative illustrations of the nature of dissipative forces?
I'm teaching a conceptual introduction to physics for American 13-15 year old students this summer.
One of the main ideas I want to hit on is the relationship between energy conservation, ...
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3answers
480 views
What does it take to derive the ideal gas law in themodynamics?
How can the ideal gas law be derived from the following assumptions/observations/postulates, and these only ?
I'm able to measure pressure $P$ and volume $V$ for gases.
I notices that if ...
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2answers
372 views
How long will my fridge magnet stay attached to my fridge?
I have a 500 gram neodymium magnet. It is stuck to my fridge. With the constant pull of gravity trying to pull it off how long would it stay attached to fridge? For arguments sake lets say my fridge ...
3
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2answers
289 views
Does the global financial system violate laws of thermodynamics and energy conservation?
For the past years I am thinking about relating concepts from physics to concepts from economics. Especially since the financial crisis made obvious the instability of the financial system I ask ...
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2answers
249 views
Will the water added to an ice piece freeze?
Water, at room temperature is poured into a hole made of a block of melting ice(kept at room temperature).I was wondering if the water will ever freeze?
Thank you.
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1answer
653 views
Why did my liquid soda freeze once I pulled it out of the fridge and opened it?
This isn't a duplicate to "Why did my liquid soda freeze once I pulled it out of the fridge?". My question is why soda froze after it was opened. Opening a can or bottle seems to have a larger effect ...
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3answers
2k views
Pressure inside a light bulb
It is known that nowadays most light bulbs are filled with gas in order to minimise the evaporation rate of tungsten. Is e.g. argon put into the bulb with reduced pressure, so that the glass doesn't ...
3
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3answers
265 views
How can my water cool down more quickly?
I have a cup and I can only pour hot water inside, I wanna know whether the heat will dissipate more quickly with more water or less water?
How about the occasion when my cup is well covered?
3
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1answer
130 views
Why aren't two systems in thermal equilibrium the same as one system?
I am reading Molecular Driving Forces, 2nd ed., by Dill & Bromberg.
On page 53, example 3.9, we consider why energy exchanges between two systems from the point of view of the 2nd law.
We ...
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2answers
130 views
Question about termodynamic conjugate quantities
I've come across the Onsager reciprocal principle. It's almost clear, except for thermodynamic conjugate quantities - what's that, physical meaning (except the formal definitions: $X_i = ...
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3answers
760 views
How can I determine density of a gas only given temperature?
I have a homework problem where exhaust is traveling through an exhaust system (assumed to be air for simplicity) from and engine and then released into the atmosphere. The exhaust is at a temperature ...
